1. Field of the Invention
This invention relates generally to circular staircases and particularly to a circular staircase having inside and outside stringers constructed from a plurality of parallelogram shaped blocks connected end to end.
2. Prior Art
A circular staircase has three main components, an inside and outside stringer and the treads. Typically, a stringer extends from a first floor to a second floor through an angle of about 90.degree.. The radius of the curve of a stringer may vary. In the simplest example, both the inside and outside stringers curve along constant radii and the tread length is also constant throughout the staircases. Handrails, supported by balusters extending upward from each stringer, complete the staircase.
The greatest expense of making a circular staircase, both in labor and materials, is constructing the stringers. One prior art method of building a stringer is to bend a board around a large cylindrical form. Additional layers of boards are glued on to create a curved, laminated beam representing a portion of a helix. Right angle cylindrical forms are used to produce a stringer, which when installed, will have side edges perpendicular to the floor. Standard sized stringers are available from staircase manufacturers. However, deviations from the standard sizes require custom built stringers, further increasing the expense of the staircase. Another shortcoming of the prior art methods of construction is that the stringers are expensive to ship owing to their large size and awkward configuration. Even when the stringers are constructed on-site, the necessary cylindrical forms must be available at the site.
A modular design for spiral staircase, rather than circular staircase, construction is shown in Hughes, Jr., U.S. Pat. No. 3,491,498. In Hughs, Jr., the treads are sandwiched between column sections. The column sections are truncated parallel to the floor with the axis of each section being slanted in relation to the floor.
However, the technology developed by Hughes for a spiral staircase is not readily applied to a circular staircase for several reasons. A supporting columnn of a spiral staircase turns with a tight radius through an angle of about 270.degree. with an upward slope of about 55.degree.. Given the steep slope of the column of spiral staircase, a force normal to a tread will exert significant compressional force on the column and will have much smaller component exerting torque on the individual column sections. Conversely, the stringers of a circular staircase turn with a much larger radius through an angle of about 90.degree.. The upward slope of an outside stringer is typically 31.degree.. It can be readily seen that a force acting normal to a tread in a circular staircase will exert significantly greater torque on the individual segments. If segments with horizontal ends are used to construct the circular staircase, much greater strain would be put on the tensioning cables as the slope of the supporting columns decreases. Most building codes require that a circular staircase be able to support 100 pounds per square foot.
Traditional circular staircase stringers have the shape of a curved beam with approximately a 13" height. The sides of a stringer are perpendicular to the floor. Hughes method, which employs segments with horizontal ends, when applied to a beam shaped stringer, would produce a discontinuous jagged surface. For example if one segment is used per tread, each segment must be rotated approximately 7.degree. from the adjoining segment. If a parallelogram sided segment is turned 7.degree. in a horizontal plane, the horizontal ends would not align. The less the slope of the stringer, the longer the horizontal ends and the greater the effect the rotation would have on alignment. Therefore, not only are there stability problems associated with building a circular staircase with the method disclosed in Hughes, U.S. Pat. No. 3,491,498, but the resuld would not simulate the aesthetically pleasing features of a traditional circular staircase.